Conventionally, a CRT or liquid crystal display has been used for a display terminal of so-called images such as a character, a static image and a dynamic image. Although these may display and rewrite digital data instantaneously, it is difficult to always carry the device and there are many drawbacks such that, for example, eyes may be fatigued on an extended activity or display on a power-off is hardly made. Meanwhile, when a character or a static image is distributed or stored as a document, etc., it is recorded on a paper medium by a printer. The paper medium has been widely used as a so-called hard copy. The hard copy is easier to read than a display, is not easy to cause a fatigue, and may be read in a free posture. Furthermore, it may have a feature of being lightweight so that it can be carried freely. However, a hardcopy is disposed after use thereof and recycled and a problem in terms of resource saving remains in the recycle thereof since a large amount of labor and cost are required.
A need for a rewritable and paper-like display medium having both merits of the above-mentioned display and hardcopy has been high and a display medium using a polymer-dispersed liquid crystal, a bi-stable cholesteric liquid crystal, an electrochromic element, an electrophoretic element, or the like has still been proposed and has been drawing attention as a display medium that is a reflection display type one so as to provide a bright display and may be a memory. Among these, use of an electrophoretic element may be excellent in terms of a display quality and electric power consumption at an displaying operation, which is disclosed in, for example, Japanese Patent Application Publication No. 5-173194 and Japanese Patent No. 2612472. For an electrophoretic display medium, a dispersion liquid in which plural electrophoretic particles are dispersed in a colored dispersion medium which have a color different from the color of the dispersion medium is enclosed between a pair of transparent electrodes. In this case, the surfaces of the electrophoretic particles (also simply referred to as “phoresis particles” in this section) are charged in the dispersion medium and the particles may be moved by forming an electric field between the transparent electrodes. For example, when a charge opposite to the charge of the phoresis particles is provided on one of the transparent electrodes, the phoresis particles are attracted to it so as to deposit near the electrode, whereby the color of the phoresis particles is observed. On the other hand, when the same charge as the charge of the phoresis particles is provided, the phoresis particles move to the opposite side, whereby the color of the dispersion medium is observed. Thus, each kind of display may be conducted by applying a principle such that the observed color is changed by changing the charge on the electrode surface.
Charging of an electrophoretic particle greatly depends on a functional group on a particle surface. That is, since an ionic polar group of a particle surface is ionized and accordingly has a charge, a particle may be charged by positively providing an ionic polar group to the particle. For a positively charging polar group, an amino group is known and for negatively charging polar group, a carboxyl group, a sulfonyl group, a phosphonyl group, etc., are known.
It has been widely known that when an electrophretic particle has at least a polymer component, a polymer containing a monomer unit having an amino group may be used as a positively charged particle. For example, in the case of an addition polymer of a monomer having a vinyl group, the examples of a monomer having an amino group include acrylates and methacrylates such as N-methylaminoethyl (meth)acrylate, N-ethylaminoethyl (meth)acrylate, N,N-dimethylaminoethyl (meth)acrylate, N,N-diethylaminoethyl (meth)acrylate, N,N-dibutylaminoethyl acrylate, N,N-di-tert-butylaminoethyl acrylate, 2-N-piperidylethyl (meth)acrylate, N-phenylaminoethyl methacrylate, and N,N-diphenylaminoethyl methacrylate; styrene derivatives such as dimethylaminostyrene, N-methylamionoethylstyrene, dimethylaminoethoxystyrene, diphenylaminoethylstyrene, and N-phenylaminoethylstyrene; vinylpyridine-type compounds such as 2-vinylpyridine, 4-vinylpyridine, and 2-vinyl-6-methylpyridine; and the like.
In an electrophoretic particle, the more the amount of the charge is, the better the phoresis velocity or the responsiveness at a lower electric field is, which is preferable in terms of improvement of a display-switching rate or reduction of a driving voltage. For the polymer having an amino group, it is preferable that the amino group has a strong basicity. For such a monomer having a strong basicity, compounds shown in the following general formula (I) are provided.

[In the formula, R represents a hydrogen atom or a methyl group.]
Examples of use thereof are disclosed in, for example, Japanese Patent Application publication No. 2004-054248 and Japanese Patent Application Publication No. 2006-018236, although the purposes thereof are different.
As described above, the electrophoretic particles have been proposed, and in particular, the compounds shown in general formula (I) have a strong basicity and a sufficient charged property but the hydrophilicity thereof may be too high due to the strong basicity and it may have no affinity with a non-polar dispersion medium in an electrophoretic display medium, may be difficult to be stably dispersed in the dispersion medium and may be easy to cause aggregation or sedimentation thereof in the dispersion liquid. For a particle of electrophoretic display medium, there may be a difficult problem of the balance between a high charged property and a high dispersion stability.